1. Field of the Invention
The present invention relates to a process to recover sulfur values from waste gas emissions, e.g., a flue gas or tail gas, by use of an absorbent containing sulfites followed by a liquid phase Claus reaction to produce sulfur as a useable by-product.
2. Description of the Prior Art
U.S. Pat. No. 4,048,293 to P. Renault et al. describes a process in which a feed gas containing sulfur dioxide (SO.sub.2) is contacted with an aqueous ammonia and/or ammonium sulfite absorbent to remove SO.sub.2 from the gas, the SO.sub.2 -rich absorbent is heated producing a gas stream comprising NH.sub.3, SO.sub.2 and steam, and hydrogen sulfide (H.sub.2 S) is reacted with the gas stream containing NH.sub.3, SO.sub.2 and steam in a high temperature liquid phase Claus reactor, preferably in the presence of an organic solvent, at a temperature of 100.degree. C. to 200.degree. C. to produce sulfur. The liquid phase from the heating step in the Renault et al. process may be discharged from the plant, but is preferably fed to another reactor for reaction of the ammonium sulfate contained therein with a reducing agent yielding a second gas stream comprising SO.sub.2 and ammonia which is then fed to the high temperature liquid phase Claus reactor (Col. 1, lines 51-56). In addition, the excess H.sub.2 S from the Claus reactor tail gas is oxidized catalytically to form SO.sub.2 which is fed back to the ammonia and/or ammonium sulfite absorber.
The process shown in the Renault et al. patent relies upon a substantially total thermal decomposition of the SO.sub.2 -rich absorbent in its stripping step yielding NH.sub.3 (which is derived from the cation, NH.sub.4.sup.+, of the absorbent and which is recycled to the absorber), SO.sub.2 and steam. Such a process requires a thermal decomposition step for the sulfite absorbent, a chemical reduction for the sulfate species that are also present, and oxidation of the ammonia-rich recycle gas.
The ammonia-containing gas stream produced in the Renault et al. process is reacted directly with hydrogen sulfide in a liquid phase Claus reactor at relatively high temperatures (e.g., 100.degree.-200.degree. C.). This has several disadvantages. The presence of such contaminants as ammonia in the Claus reactor can cause undesired side reactions rather than the desired reaction of H.sub.2 S with SO.sub.2 to yield sulfur. Also, the use of such high temperatures inhibits the substantial completion of the Claus reaction yielding a reactor tail gas effluent that contains a higher SO.sub.2 content than otherwise possible.